Pultrusion is a manufacturing process for producing continuous lengths of fiber reinforced plastic (“FRP”) structural shapes. Raw materials include a liquid resin mixture (containing resin, fillers and specialized additives) and reinforcing fibers. The process involves pulling these raw materials, rather than pushing as is the case in extrusion, through a heated steel forming die using a continuous pulling device. The reinforcement materials are in continuous forms such as rolls of fiberglass mat or doffs of fiberglass roving. The two ways to impregnate, or “wet out”, the glass are the Open bath process and resin injection. Commercial resins which have typically been used in pultrusion processes include polyester, vinyl esters, phenolics, and epoxy compounds. These resins usually have very long gel times and can be run in an open bath process in which the reinforcing fibers are soaked in a bath of resin and the excess resin is scraped off by a series of pre-form plates and at the die entrance. As the wetted fibers enter the die, the excess resin is squeezed through and off the reinforcing fibers. The pressure rise in the die inlet helps to enhance fiber wet-out and to suppress void formation. As the saturated reinforcements are pulled through the die, the gelation (or hardening) of the resin is initiated by the heat from the die and a rigid, cured profile that corresponds to the shape of the die is formed.
For resin systems like polyurethanes, which have fast gel times and short pot lives the resin injection process is generally used. In the injection process, the reinforcement materials are passed through a small closed box which is usually attached to the die or may be part of the die. The resin is injected under pressure through ports in the box to impregnate the reinforcement materials. Resin injection boxes are designed to minimize resin volume and resin residence time inside the box. There are a number of different resin injection box designs in the literature all of which have the common features of an angled or tapered design and an exit profile matching the shape of the die entrance.
With respect to polyurethane pultrusion, U.S. Pat. No. 6,420,493, issued to Ryckis-Kite et al., discloses a two component chemically thermoset resin for use in composite manufacturing processes. The polyisocyanate component and the polyol component are used in amounts such that the OH/NCO equivalent ratio is from 1:1 to 1:2. U.S. Pat. No. 6,420,493 requires 10%-40% of a polyester polyol. Use of from 5 to 20 wt % of a hydroxyl terminated vegetable oil is also disclosed. In the isocyanate component, it is preferred that at least 15 wt % be an aliphatic polyisocyanate.
Cheolas et al., in U.S. Pat. No. 6,793,855, teaches polyisocyanurate systems, pultrusion of those systems to produce reinforced polyisocyanurate matrix composites and the composites produced by that pultrusion process. The polyisocyanurate systems of Cheolas et al. include a polyol component, an optional chain extender, and an isocyanate. The polyisocyanurate systems are said to have extended initiation times of from about 5 minutes to about 30 minutes at room temperature and to be capable of snap curing. Cheolas et al. also teaches that substantial polymerization of the polyurethane takes place in the impregnation die.
U.S. Pat. No. 7,056,976 (Joshi et al.) also discloses polyisocyanate-based reaction systems, a pultrusion process using those systems to produce reinforced matrix composites and composites produced by that pultrusion process. The polyisocyanate-based systems are mixed activated reaction systems that include a polyol composition, an optional chain extender or crosslinker and a polyisocyanate. These polyisocyanate-based systems are said to exhibit improved processing characteristics in the manufacture of fiber reinforced thermoset composites via reactive pultrusion. Joshi et al. teaches that gel time is the key parameter in polyurethane pultrusion processes.
Pultrusion has been used to produce exterior windows, doors and other materials used in construction that require good surface quality and weather resistance. One difficulty that has been encountered with the known polyurethane pultrusion systems has been inadequate adhesion of paint, coatings, and other protective layers to polyurethane composites produced by a pultrusion process.
A market that is particularly suited for the use of composites made by polyurethane pultrusion is the commercial window market because the physical properties of polyurethanes are comparable to those of aluminum while also providing a better insulation value than aluminum. Polyurethane composite materials can match the strength of aluminum in large commercial window applications. However, in order to be used in such applications, it is necessary to satisfy the specifications for surface coatings on parts that have been set by the American Architectural Manufacturers Association (AAMA). These AAMA requirements can be satisfied by applying either powder or wet coatings to aluminum. However, powder coatings are beyond the glass transition temperature of polyurethanes so coating a polyurethane composite with a powder coating it is not an option for meeting the requirements of the AAMA standards.
To date, surface coatings which meet AAMA requirements that can be applied to a polyurethane composite require the polyurethane composite to be aggressively pretreated. This pretreatment typically has been a flame treatment and or a plasma treatment. Both procedures add cost and require additional work to ensure that the polyurethane composite will satisfy the AAMA specifications.
It would therefore be advantageous to develop a polyurethane formulation that could be used in a pultrusion process to produce a polyurethane composite to which a coating or protective layer could be readily applied without the need for extensive pretreatment by procedures such as flame or plasma treatment.
The solution to this problem of coating a polyurethane composite made by a pultrusion process is complicated by the fact that the commercial systems used to produce polyurethane composites by a pultrusion process must include an internal release agent so that the part being formed in the pultrusion process can be pulled through the pultrusion die without sticking to the die. It is this use of such internal release agents that is believed to necessitate the extensive pretreatment of a composite article in order to achieve adequate adhesion of a coating or protective layer.
Therefore, a need exists in the art for polyurethane formulations suitable for use in pultrusion processes to produce polyurethane reinforced composites to which paints, coatings and protective layers can adhere without the need for extensive pre-treatment.